(1) Field of the Invention
The present invention relates to a method for breaking a conducting path formed on or in a printed-circuit board, and more particularly to a method for breaking a conducting path formed on or in a printed-circuit board, that is highly integrated and multiple-layered, by irradiating a laser beam onto the conducting path. The present invention is also concerned with a laser system using the above method. The present invention also relates to a printed-circuit board modified by the method.
Recently, difficulties in modifying the printed-circuit board such as breaking the conducting path according to design changes has increased as the printed-circuit board is integrated more highly. Therefore, it is required to reduce the difficulties in modifying the printed-circuit board such as breaking the conducting path.
(2) Description of the Prior Art
FIG. 1 shows a conventional method for cutting a wiring pattern 101 on a multiple-layered printed-circuit board 10 with a round bar 2. The round bar 2 is used for cutting wiring patterns by a mechanical process. In this case, the round bar 2 contacts with and cuts the wiring pattern 101. In addition, as shown in FIG. 1, the round bar 2 with a radius .phi. of 0.6 mm contacts with and damages a wiring pattern 112 adjacent to the wiring pattern 101 spaced with a narrow gap 114 between the wiring pattern 112 and the wiring pattern 101, when the round bar 2 cuts the wiring pattern 101.
FIGS. 2A and 2B show a conventional method in which a wiring pattern 102 located at a surface layer 12 of the printed-circuit board 10 is cut at a point 114 with the conventional round bar 2. A plan view of the wiring pattern 102 to be cut with the round bar 2 is shown in FIG. 2A and a transverse sectional view of the printed-circuit board 10 is shown in FIG. 2B. When another wiring pattern 116, called an internal wiring pattern 116 hereinafter, locates at an internal layer 14 of the printed-circuit board 10 and below the wiring pattern 102, the round bar 2 may contact with and damage the internal wiring pattern 116.
Another exemplary case is shown in FIGS. 3A and 3B in which a wiring pattern 103 located within the internal layer 14 which expands below the surface layer 12 is cut with the round bar 2. A plan view of the wiring pattern 103 to be cut is shown in FIG. 3A and a transverse sectional view of the wiring pattern 103 located within the internal layer 14 is shown in FIG. 3B. In this case, wiring patterns 118 are located at the surface layer 12 and above the wiring pattern 103, as shown in FIG. 3. Therefore the round bar 2 may contact with and damage the wiring patterns 118 at the surface layer 12 when the round bar 2 cuts the wiring pattern 103 within the internal layer 14.
FIGS. 4A through 4D show conventional alternatives to cutting a wiring pattern 104 located below a widely extended conducting layer 120 having a large area, which is present within the multiple-layered printed-circuit board 10, to avoid damaging the widely extended conducting layer 120. The widely extended conducting layer 120 is referred to hereinafter as a "plain" 120. In FIG. 4A, the wiring pattern 104 to be cut is located below the plain 120.
One method for changing the design of the printed-circuit board to prevent the plain 120 from being damaged is shown in FIG. 4B. In this case, the wiring pattern 104 to be cut, which exists below the plain before the design change, as shown in FIG. 4A, can be replaced with a newly formed pattern 122 located above the plain 120. The newly formed pattern 122 is called a "bypassing pattern" hereinafter. The round bar 2 can cut the bypassing pattern 122, without interfering with the plain 120, because the bypassing pattern 122 provides an appropriate portion to be cut above the plain 120. The above-described design change has been used in the conventional method for preventing the plain 120 from being damaged.
In FIGS. 4C and 4D, an escaping pattern 124 for passing through the plain 120 is formed inside of the plain 120. FIG. 4C is a transverse sectional view of the escaping pattern 124 formed within the multiple-layered printed-circuit board 10. A plan view of the plain 120 is shown in FIG. 4D. With the escaping pattern 124, the plain 120 is prevented from being damaged by cutting of the wiring pattern 104 below the plain 120 with the round bar 2, because the round bar 2 can reach the wiring pattern 104 to be cut through the escaping pattern 124 without contacting with the plain 120.
FIG. 5 shows a conductivity tester 4 for checking a conductivity of a conductor such as the wiring pattern 105. As shown in FIG. 5, the conductive wiring pattern 105 is broken at a point 114 on the conductive wiring pattern 105 connecting one land 20A with another land 20B. Two terminals 6A and 6B of the conductive tester 4 are contacted with the lands 20A and 20B, respectively, to check whether the wiring pattern 105 is broken, or cut, with the round bar 2.
However, the following problems may occur in the above-described conventional methods for breaking the conducting path.
A first problem is that the wiring pattern 112 adjacent to the wiring pattern 101 to be cut with the round bar 2, as shown in FIG. 1, could be contacted and damaged with the round bar 2. This is due to the fact that the radius of the round bar 2 does not correspond to the narrow pitch 114 between the wiring patterns 101 and 112 on the highly integrated printed-circuit board 10.
A second problem is that the internal wiring pattern 116 may be damaged by cutting of the surface wiring pattern 102 with the round bar 2, as shown in FIG. 2, when the internal wiring pattern 116 is located below the surface wiring pattern 102 to be cut. When the surface wiring pattern is cut with the round bar 2, it is required to adjust a depth of cutting so as not to damage the internal wiring pattern 116. Furthermore, since clearances between layers, such as the surface layer 12 of the printed-circuit board 10 and the internal layer 14, are becoming smaller as the printed-circuit board 10 is more multiple-layered, it is required to adjust the depth of cutting more precisely.
A third problem is that the narrow pitch 114 between the surface wiring patterns 118 causes the round bar 2 to contact with and damage the surface wiring patterns 118 in cutting the internal wiring pattern 103, as shown in FIG. 3B. As the pitch 114 becomes narrower and narrower in accordance with the highly integrated printed-circuit board 10, it is more difficult to cut only the internal wiring pattern 103 without damaging the surface wiring patterns 118.
A forth problem is that a design change of the printed-circuit board 10, which wastes time and increases costs, is required when the plain 120 is located above the wiring pattern 104 to be cut by the round bar 2, as shown in FIG. 4B. For example, the bypassing wiring pattern 122 is formed on the basis of the design change to break the conducting path on the printed-circuit board 10.
Another example of a design change to prevent the plain 120 from being damaged by the round bar 2 is shown in FIG. 4C. In this case, the escaping pattern 124 is formed within the plain 120, and the round bar 2 passing through the escaping pattern 124 cuts the wiring pattern 104.
In designing a circuit comprised of elements which operate at high speed, signal lines or wiring patterns are desirably located so as to pass above the plain 120 when the printed-circuit board 10 has the plain 120. This arrangement has an advantage that a characteristic impedance matching of the circuit is achieved. However, when the non-conductive escaping pattern 124 is formed within the plain 120 as described above, the signal lines or wiring patterns fail to pass above the conductive plain 120. This causes a fifth problem that the characteristic impedance matching of the circuit is not achieved when the escaping pattern 124 is formed within the plain 120 for preventing the plain 120 from being damaged by the round bar 2.
A sixth problem occurs when the conducting path is broken by cutting the wiring pattern 105 connecting two lands 20A and 20B at the point 114 with the round bar 2. In this case, it is required to find out whether the land 20A conducts to the land 20B to detect whether the conducting path is successfully broken. As is shown in FIG. 5, this checking process, for example, is executed with aid of the conductivity tester 4. In use of the conductivity tester 4, it is required to find positions of the lands 20A and 20B, and also contact the lands 20A and 20B with the terminals 6A and 6B, respectively. Such a testing process requires elaborate and time-consuming work.
Furthermore, in the conventional methods described above for breaking the conducting path by cutting the wiring pattern with the round bar 2, a visual inspection based on a comparison between a drawing of the printed-circuit board and an actual multiple-layered printed-circuit board 10 is required to find the wiring pattern to be cut. It is a seventh problem that the visual inspection is not an efficient method finding any type of wiring pattern. For example, the conducting path is often covered with a layer such as a resist layer, and thus, it takes much time to find an appropriate portion to be cut. In addition, if the wiring pattern to be cut was present in the internal layer of the multi-layered printed-circuit board 10, it is more difficult to find the wiring pattern to be cut.